A closed-cell plastics foam is hereinafter defined as one in which the majority of the cells are closed. Typically, a closed-cell foam of a reasonable quality has at least 70% closed cells, and a closed-cell foam of a good quality has at least 85% closed cells, although this may be less in thin sheets (e.g., 1 mm or less).
A conventional starting material used in manufacturing low density polyolefin foam is a polyolefin resin such as a highly-branched low density polyethylene (LDPE), typically manufactured by a high-pressure radical polymerization process, such as, for example, tubular autoclave. In this process, the monomers are mixed thoroughly under high pressure and at a high heat that induces polymerization, and forms a polyethylene resin.
To produce a foam, this polyethylene resin, typically in pelletized form, is then plasticized in a screw extruder, and mixed with a blowing agent. When the material is extruded into, for example, a sheet form, the blowing agent expands, which produces a large number of small bubbles.
Where the polyethylene foam is used as a packaging material, while some physical strength characteristics are important, cost is very important, and over many years, much effort has been put into reducing cost.
To date, a majority of the effect in reducing the cost has been directed towards lowering the foam density and, by conventional techniques, it is possible to produce a material having a density as low as 15 to 20 kg/m3.
Alternative materials have been considered in an attempt to further reduce the cost. In particular, a number of attempts have been made to use a linear low density polyolefin such as a linear low density polyethylene (LLDPE). LLDPE is typically manufactured at a lower pressure and a lower heat, using catalysts. At this time, LLDPE is approximately 10% less expensive than LDPEs.
However, conventional Ziegler-Natta LLDPE, hereinafter defined as LLDPE manufactured without using a metallocene (or a similar substance) as a catalyst, contains predominantly linear polymer chains with irregular short-chain branching and no substantial long-chain branching. Conventional Ziegler-Natta LLDPE has a narrow molecular weight distribution (MWD), which results in the material having poor foaming characteristics, specifically having a lower melt strength.
LLDPE may be produced by using different catalyst systems, (e.g., a metallocene catalyst), which can lead to producing polymers having substantial long-chain side branching. This long-chain side branching results in a material having a broad MWD and a higher melt strength. This method consequently has a higher cost. Such LLDPE may be blended with LDPE to produce a material that may be foamed to the required low density. Metallocene catalyzation, however, is an expensive process, and the cost of such modified LLDPE results in a material that is in fact more expensive than LDPE itself.
Alternatively, it is possible to add cross-linking agents such as organic peroxides to the LLDPE/LDPE mix, which again increase the extent of long-chain branching and the melt strength of the mix. However, these additional compounds are also expensive.
Attempts have been made to produce a foam from a blend of small quantities of conventional Ziegler-Natta LLDPE with LDPE, but it has been found that the melt strength of the blend is increased and so difficulty is encountered in achieving as low of density as is obtained using 100% LDPE, again resulting in a higher cost outweighing the savings resulting from using LLDPE.
Thus, it would be desirable to have a foam that overcomes at least some of the above-noted shortcomings of existing LDPE foam blends.